Gas monitors and gas detection devices are well-known as analytical tools and for detecting the presence of a particular gas in the workplace. The known devices operate on various principles employing optical, chemical, chromatographic, electrochemical, and catalytic techniques to detect specific gases. Optical devices detect infrared absorption spectra, are expensive and not portable. They are not capable of continuous monitoring. Chemical reaction devices, such as calorimetric systems and gas chromatography analyzers are also expensive, bulky and cannot continuously monitor the environment. Electrochemical real time gas detection usually occurs in a cell containing gas dissolved in an electrolyte. This method of detection is not highly selective, the electrolyte is susceptible to freezing and the process consumes relatively large amounts of power. Catalytic sensors generally rely on a thermal sensor that measures the temperature change of a catalytic material or a semiconductor catalytic sensor. Semiconductor sensors used in such detectors include zirconium oxide, titanium oxide, indium oxide, tin oxide, tungsten oxide, platinum and palladium doped metal oxides, and mixtures of them. Impedance changes in the oxide indicate the occurrence of a catalytic reaction.
All of the cited gas detectors are large and/or consume relatively large amounts of electrical power. As a result, these detectors, if portable at all, require rather large and heavy batteries as portable power supplies. Therefore, these gas detectors are not conveniently used by personnel in the way film badges and other dosimeters are. In fact, the smallest portable gas detector presently available is about the size of a package of cigarettes and weighs about one half pound (225 gms.) It is highly desirable to make a smaller and lighter weight gas selective detector that may be continuously and conveniently worn by an individual.
Silicon field effect transistors having sensitivities to particular gases have been reported by Lundstrom et al. in Applied Physics Letters, Vol. 26, No. 2, Jan. 15, 1975. More recently, suspended gate chemically sensitive field effect transistors have been disclosed in U.S. Pat. Nos. 4,411,741 and 4,514,263 to Janata and 4,456,522 and 4,486,292 to Blackburn. Those patents are incorporated in this application by reference. The power consumption of these so-called CHEMFETS is very small so that a lightweight battery can power a CHEMFET for a long time.